Liquid crystals in their various ordered and disordered phases have been shown to possess large optical nonlinearities over wide temperature and spectral ranges. Consequently, almost all types of nonlinear optical phenomena have been observed. In particular, nonlinear propagation and optical limiting, stimulated backscattering and phase conjugation have been observed in bulk, thin-film and liquid cells. In the case of optical limiting applications, the threshold for such nonlinear effect in bulk films of a class of isotropic liquid crystals is very low and ranks among the lowest of all known nonlinear optical materials. However, in tightly focused geometries, thresholds begin to increase drastically. Such an increase is attributed to the decreased interaction region in tightly focused geometries. In a reported experiment, it has been demonstrated that the threshold for optical limiting of nanosecond visible-laser pulses could be considerably reduced if guided-wave geometry is employed, and in tightly focused geometries, the low threshold for optical limiting appeared to hold.
Optical limiting action with the ordered-phase liquid crystals, such as nematics, has been studied in various contexts. The application of these effects to practical devices, however, suffers a fundamental limitation imposed by the large scattering loss in the nematic phase.
Typically, the orientational fluctuation-induced scattering loss amounts to about 20 db/cm (about 100 cm.sup.-1) in nematics and slightly less in smectics. They are, therefore, usually assembled in thin-film forms for nonlinear optical applications, with the laser-liquid crystal interaction region limited to less than 10.sup.2 .mu.m. See: I. C. Khoo, S. T. Wu: Optics and Nonlinear Optics of Liquid Crystals (World Scientific, Singapore 1993); I. C. Khoo, Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena, Wiley Interscience, 1995; and I. C. Khoo et al.; Special Issue on Optical Switches Limiters and Discriminators, Int. J. Nonlinear Opt. Phys. 3, pp. 559-575, (1993) and I. C. Khoo et al., Opt. Lett. 19, 530 (1994).
On the other hand, orientational and density-fluctuation-induced scattering losses are considerably reduced in the isotropic phase (&lt;&lt;1 cm.sup.-1), thus permitting interaction geometries involving much longer optical path lengths. Furthermore, isotropic liquid crystals have also been shown to possess sizable (comparable to the numatic and smectic phase) optical nonlinearities on the nanosecond time scale, and, generally, respond faster than the ordered phase.
In recent papers, Khoo et al. Optics Letters, 19, pp. 530-532 (1994) and Applied Physics, B59, pp. 573-580 (1994), Applicant has reported the results of a series of experiments carried out on a particular class of commercially available isotropic liquid crystal mixtures (TM74A, from EA Chemicals, New York) which is henceforth called ILC. The chemical structures of the molecular constituents of ILC are shown in FIG. 1. This material has also previously been shown to exhibit stimulated backscattering and phase-conjugation and optical limiting effects in conjunction with nanosecond Nd:YAG (0.523 .mu.m) laser pulses.
On nanosecond and longer time scales, optical nonlinearities of isotropic-phase, liquid crystals suitable for practical applications are laser-induced molecular orientation, thermal and density effects. These nonlinearities do not directly involve the electronic structures of the molecular constituents. They are, therefore, adequately described by a model in which the liquid-crystalline molecules are anisotropic. In the isotropic phase, molecules are randomly distributed and theoretical modelling of the nonlinear optical phenomena are considerably simpler as one may use the averaged or effective values for anisotropic physical parameters.
The optical nonlinearities are described by the following equations.
Orientational: Isotropic phase ##EQU1##
Thermal ##EQU2##
Density ##EQU3## where .gamma. and .eta. are viscosity coefficients, Q is the order parameter, .DELTA..sub..chi. the susceptibility anisotropy, .alpha. the absorption constant, .rho..sub.0 the density, C.sub.p C.sub.v the specific heats, .upsilon..sub.s the sound velocity, B the bulk modulus, .lambda..sub.T thermal conductivity, .gamma..sup.e the electrostrictive coefficient, E the optical electric field, and A=a(T-T*) is a parameter characterizing the orientational response of isotropic liquid crystals near the phase transition temperature T.sub.c (.apprxeq.T*).
Equation (1) describes the molecular-orientation order induced by the laser in the isotropic phase where the equilibrium value of the order parameter is zero. It can be shown that such an induced-ordering effect gives rise to a third-order nonlinear polarization, i.e., an intensity refractive-index change. Studies have shown that the third-order nonlinear susceptibility .chi..sup.(3) is on the order of 10.sup.-10 esu and is highly dependent on the temperature vicinity of T.sub.c. Equally important to note is the dependence of the response-time constant .tau. on the temperature .tau.=.eta./a(T-T*).
The closer it is to T.sub.c, the longer is the response time. Response times ranging from a few nanoseconds to hundreds of nanoseconds have been observed. Therefore, the role played by laser-induced molecular reorientation in the optical limiting action of nanosecond-laser pulses is also highly dependent on the laser-field-polarization state as well as the temperature. For (cylindrical) guided-wave devices intended for nanosecond response, such orientational nonlinearity is not very useful.
Equation (2), which is coupled to (3) for the density fluctuation, describes the laser-induced temperature increase in the liquid crystal as a result of (linear or nonlinear photoabsorption). The amount of temperature rise and the resulting refractive-index change depends on the interaction geometry, e.g., the focused laser-spot size and the material's physical dimensions and the conductivities of boundary materials.
Equation (3) describes the laser-induced density change associated with the electrostrictive effect and the temperature rise. Note that any density wave thus created will propagate away from the region of interaction with the sound velocity; the Brillouin damping constant .tau..sub.B for the decay of the density component is also dependent on the dimension of the interaction region.
For the typical geometry and laser-spot sizes involved in the processes under study, the typical Brillouin response time .tau..sub.B is on the order of a few to tens of nanoseconds. It is not sensitive to the temperature.
In recent studies of self-defocusing phenomena, it has been demonstrated that a laser-induced density change, and the accompanying change in refractive index, can lead to a low-threshold optical limiting action. The experimentally observed threshold for a bulk film, using a self-limiting set up as shown in FIG. 2, are plotted in FIG. 3. FIG. 3 plots observed optical limiting-threshold fluence (in units of laser energy per unit area) as a function of the focused laser-spot diameter on the ILC film, and shows large increases at small diameters. Notice that as the input-laser beam-spot diameter is reduced to below 40 .mu.m, the threshold laser fluence (in J/cm.sup.2) begins to increase drastically. This is attributed to the reduced interaction region between the laser pulse and the laser-induced thermal density index change in a tightly focused geometry.
In sum, liquid crystals in their various ordered and disordered phases have been shown to possess large optical nonlinearities over a wide temperature and spectral range. Consequently, almost all types of nonlinear optical phenomena have been observed. In particular, nonlinear propagation and optical limiting, stimulated back scattering and phase conjugation have all been observed in bulk thin film or liquid cells. It is also known that a nonlinear optical response of a material will be greatly enhanced (and the efficiency of a nonlinear phenomena enhanced) if guided wave geometries are employed.
There is a need for devices which protect both the eyes and sensitive optical instruments against damage from an incident laser beam. However, liquid crystal nonlinearity thresholds exhibited by bulk thin films and/or liquid crystal cells are too high to provide a satisfactory optical limiting effect when subjected to a high intensity optical beam. As a result, liquid crystal geometries have not heretofore been used for optical protection applications which must respond in nanosecond time intervals.
Accordingly, it is an object of the invention to provide a device which protects against incident high intensity optical beams.
It is another object of this invention to provide an optical protection device which requires no external control instrumentality.
It is yet another object of this invention to provide an optical protective device that is physically small and reacts to an incident beam in a nanosecond time interval.